The payment has only been split into 2 payments since my DD1 (who will be 4 in May) was born. I would check, but you may have already received the whole payment as one lump sum when your DD1 was 18 months old.

ETA: Found a link for you http://www.immunise.health.gov.au/in...lated-payments - second paragraph from the bottom. Sorry this is short, typing one handed